Transfer texture from one mesh to another with transparency?
Transparency is a mask, a black &amp; white map (0=transparent, 1=opaque). You can store this data in the Alpha channel of an image (RGBA) or use a separate (b&amp;w) image.
Plug this channel in the Alpha slot of the Principled BSDF to have transparency. (In EEVEE you have to set the Blend Mode in the material's Settings to Alpha Clip, Alpha Hashed, or Alpha Blend to see the transparency. With brush settings Erase/Add Alpha you can paint transparency in blender.)

Baking
With the Emission node in the material you can bake the Emit color to an b&amp;w image. In Gimp or Photoshop you can add this b&amp;w image as alpha channel/tranparency layer to the diffuse color image and save it as PNG file. This file format supports  RGBA/transparency.
Back in Blender, connect the Color slot of the ImageTexture node (with the baked image) to the Base Color of the BSDF, and the Alpha to the Alpha. Done :)